1. Technical Field
The present invention relates to a position control device for a feed shaft (a table) such as those of machine tools.
2. Related Art
For full-closed control systems in which a linear scale is attached to a movable portion of a machine tool, when the movable portion is fed in a very low speed region, stick-slip occurs due to friction or elastic deformation. In order to reduce the influence of stick-slip, attempts have been made to reduce overshoot by setting high gains for a speed loop and a position loop.
FIG. 5 is a block diagram showing full-closed control in a related-art position control device. A first position detector 17 is attached to a motor 18. A differentiator 12 differentiates a position detection value Ym detected by the first position detector 17, and outputs a speed detection value Vm of the motor 18.
A position command Rc input from a host device is input to a differentiator 8 and a subtractor 1. The differentiator 8 differentiates the position command Rc, and outputs the result as a speed feedforward command Vr. The speed feedforward command Vr is differentiated by a differentiator 10, and then multiplied by a torque feedforward coefficient Ka, and the result is output as a torque feedforward command Tff.
A second position detector 21 is attached to a rotating table 19 (a feed shaft) which is driven by the motor. A position detection value Yl detected by the second position detector 21 is input to the subtractor 1 and a differentiator 11. The subtractor 1 subtracts, from the position command Rc, the position detection value Yl detected by the second position detector 21, and outputs a positional deviation Ep. An adder adds an output obtained by multiplying the positional deviation Ep by a position loop gain Kp, and the speed feedforward command Vr, and outputs a speed command Vc. A subtractor 3 subtracts, from the speed command Vc, the speed detection value Vm of the motor 18, and outputs a motor speed deviation Em.
The differentiator 11 differentiates the position detection value Yl detected by the second position detector 21, and outputs a speed detection value Vl of the rotating table 19. A subtractor 4 subtracts the speed detection value Vl of the rotating table 19 from the speed feedforward command Vr, and outputs an actual speed deviation El.
An adder 5 adds an output obtained by multiplying the actual speed deviation El by a proportional gain Pl, an output obtained by multiplying the motor speed deviation Em by a proportional gain Pm, and an output obtained by inputting the motor speed deviation Em to an integral compensator 9, and outputs a torque feedback command Tfb.
An adder 6 adds the torque feedforward command Tff and the torque feedback command Tfb, and outputs a torque command Tc. Reference numeral 15 in FIG. 5 represents various filter units for filtering the torque command and current control units.
For feed shafts having a slide surface that is slidably guided, or main shafts that are caused to rotate under braking, there are problems in that when they are operated in a very low speed region, stick-slip occurs due to friction or elastic deformation, and the machining accuracy degrades. In the related art shown in FIG. 5, even when the control target is a large-scale machine in which lost motion including a loose ball screw or flexure of a belt is present, by feeding back a speed vl of the rotating table to the speed loop, it is made possible to set the position loop gain to be high, and the follow-up properties are dramatically improved. However, if static friction is large relative to kinetic friction, stick-slip in which a standstill and an overshoot are repeated occurs in a very low speed region.
By setting an integral gain to be large, or by shortening an integral time constant to increase the response speed of switching from static friction torque to kinetic friction torque, it may be possible to suppress occurrence of stick-slip. However, when an integral gain is set to be large, or an integral time constant is shortened, during an operation in which the acceleration increases, such as, for example, in cases where a large speed command is made, the influence of a lost motion becomes large, and mechanical vibrations occur.